A common type of interior trim panel 2 (FIG. 4) or cover 20 used to cover and hide the air bag 40 (FIG. 5) in a motor vehicle may comprise a thin elastic plastic outer skin 22 having a non-glaring grained outer surface, a rigid retainer or substrate 26 and an intermediate layer of soft plastic foam 24 between the skin and substrate. In some cases the foam is omitted.
It is common practice to use vinyl, typically polyvinyl chloride (PVC), to form the thin plastic outer shell or skin 22 for the panel 2. The skin material may also comprise urethanes (e.g., PU), olefins (e.g., PP, PS, TPO, ETP-TPO), esters (e.g., COPE), styrenes (e.g., AAS, ASA) and rubbers (e.g. TPO, ETP-TPO, ABS) in various compositions.
The present invention relates to those trim panels 2 or covers 20 wherein the skin 22 is formed of a thermoplastic polymer or thermosetting resin and is provided with an invisible tear seam 30, and the substrate 26 includes one or more doors 29 that are impacted by the inflating air bag and press against the cover to separate the tear seam 30 and then swing outward to form an opening in the cover 20 for deployment of the air bag 40 into a protective position in the passenger space. These tear seams 30 are provided in various configurations or patterns with the most common having a C, H, U, or X-shape and wherein the pattern determines the number of doors required in the substrate.
To develop tearing and/or breaking, air bag doors 29 that are integrally formed with automotive trim or instrument panels 2 will sometimes include regions of weakened materials, reduced thickness or scoring, all of which are commonly referred to as “tear seams” 30. Tear seams are weakened areas designed to preferentially tear and/or break when an air bag inflates and forces the doors to open.
It is desirable that the tear seams 30 and thereby the presence of the air bag 40 be hidden from view for various reasons and heretofore, this has been accomplished in several different ways. Such a tear seam is commonly referred to as an “invisible tear seam”. One way of providing such an invisible tear seam 30 is by forming a tear seam defining groove 32 or series of depressions in the backside of the skin. This leaves a thin and thereby weakened section at the outer or appearance side of the skin that defines the tear seam 30 without outwardly revealing its presence during normal view by an ordinary vehicle occupant. Examples of such invisible air bag cover tear seams are disclosed in U.S. Pat. Nos. 5,072,967; 5,082,310; 5,316,822; 5,348,339; 5,632,914; 5,863,064 and 5,961,143.
A variety of disclosures exist directed at preweakening a skin section employed in an automotive airbag deployment system.
U.S. Pat. Nos. 4,524,894 and 4,805,390 are directed at the use of an apparatus for cutting patterns and a groove cutting machine to provide a constant thickness of material remaining after the cutting process.
U.S. Pat. Nos. 5,803,489 and 5,968,381 are directed to an air bag cover with a tear line with a predetermined wall thickness which tapers from the inside towards the outside with a width of 120–190 microns.
U.S. Pat. No. 6,308,391 discloses a method of producing V-shaped grooves machines in the surface of different materials. The V-shaped grooves have a predetermined depth relative to the surface. The V-shaped grooves are of a single angle.
Other background disclosures include U.S. application Ser. Nos. 10/027,779; 09/778,394; 10/251,897; 10/241,366; GB 2,276,354; JP 6218811; and EP 1177878.
When soft plastic or elastomeric skin materials are used as a cover skin for an air bag deployment system, it is not uncommon for the skin material that forms the invisible tear seam in the backside of the cover skin to heal or stick together if an insufficient amount of material is not removed when the tear seam is formed. Thus, the angle of the side walls of the seam is critical. Additionally, some higher powered laser weakening systems may cause degradation of the skin surface where it has been thinned or weakened, resulting in a tear seam that remains tacky. A further issue with these systems is the smoke produced, making the work area quite unpleasant.
An alternate method to laser weakening involves the use of a knife-blade which may score the backside of the cover skin to a known depth, thus providing a line of mechanical weakness or tear seam. The knife blade may be heated or excited by another source of energy such as ultrasonics to provide a reliable cut or scoring line. The scoring need not be a continuous groove but may be intermittent and vary in depth dependent on where the tearing of the seam is desired to initiate and propagate.
It is therefore desirable to provide a knife-blade design that prevents healing of the tear seam, particularly for soft elastomeric skin materials.